1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures. More particularly, the present invention is concerned with novel alkenyl-substituted phenylisothiocyanates and benzonitriles as well as liquid crystalline mixtures which contain these compounds and the use of the compounds and mixtures for electro-optical purposes.
2. Description
Liquid crystals have acquired significance primarily as dielectrics in indicating devices, as the optical properties of such substances can be influenced by an electrical voltage. Electro-optical devices based on liquid crystals are known to the person skilled in the art and can be based on various effects such as, for example, the dynamic scattering, the deformation of aligned phases (DAP type), the Schadt-Helfrich effect (rotation cell), the guest/host effect (guest/host cell) or a cholesteric-nematic phase transition (phase change cell).
In order to be suitable as dieletrics for electro-optical indicating devices liquid crystals must, however, satisfy a number of requirements. For example, they must have a good chemical stability with respect to environmental influences such as e.g. heat, moisture, air and electromagnetic radiation in the infrared, visible and ultraviolet range. Further, they should be colourless, should give a good contact and should have a nematic or cholesteric mesophase in the entire temperature range in which the liquid crystal cell is to be operated. Furthermore, the liquid crystals should have low viscosities and short response times. The latter is especially important when they are used in television apparatus. Further, a high multiplexibility and at the same time steep transmission curves are required for indicators with a high density of information. Further properties must fulfill different conditions depending on the type of cell which is used. For example, liquid crystals which are used in rotation cells should have a large positive anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;0, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto), and liquid crystals which are used in guest/host cells should have a large positive or negative anisotropy of the dielectric constants. Moreover, in both cases a low threshold potential and a conductivity which is as low as possible are desirable.
As it is generally not possible to achieve all of the desired and to some extent inconsistent properties with a single compound, attempts have mainly been made to optimize the properties for the respective uses by mixing several components. Having regard to the high requirements in the case of uses with a high density of information and especially in the case of uses in television apparatus and autos, there exists, however, a need for further components with which especially the response times and the steepness of the transmission curves can be improved further.
The present invention provides novel liquid crystalline compounds and mixtures possessing such desired properties.